1. Field of the Invention
The present invention relates to an electronic device, and more particularly concerns an improved electronic device in which an electromagnetic shield is realized at low costs.
2. Description of the Background Art
First, an explanation will be given of terms used in the present specification. In the present specification, a metal foil refers to a metal member having such a thickness to be easily bent by a manual job, and a metal plate refers to that having such a thickness to be hardly bent by a manual job.
FIGS. 38 and 39 are exploded assembly drawings that show an inner structure of a conventional electronic device that forms the background of the present invention. This electronic device 150 is designed as a CRT display monitor, and provided with a main body 151 and a case 95. The main body 151 is provided with a CRT 4, electromagnetic shield members 1, 2, 3, 5, 6 and 10, a printed circuit board 14 and a CRT circuit board 15. Each of the electromagnetic shield members 1, 2, 3, 5, 6 and 10 is made of a metal plate of aluminum or iron, and they are connected to each other in such a manner as to surround the CRT 4, the printed circuit board 14 and the CRT circuit board 15, and further electrically grounded. Thus, the electromagnetic shield members 1, 2, 3, 5, 6 and 10 cooperatively function as an electromagnetic shield for suppressing electromagnetic radiation from the CRT 4 and various circuits to the exterior, i.e. for suppressing electromagnetic interference.
The electromagnetic shield members 1, 2, 3, 5, 6 and 10 are respectively referred to as 1: POWER-LEFT, 2: SHIELD-VIDEO, 3: REAR-PLATE, 5: SHIELD-RIGHT, 6: EMS, and 10: TOP-SHIELD. Except for the electromagnetic shield members 2 and 10, the electromagnetic shield members 1, 3, 5 and 6 not only suppress the electromagnetic interference, but also maintain mechanical strength so as to withstand vibration and impact that are imposed as the electronic device 150 is transported.
The printed circuit board 14 on which a video amplifier for amplifying image signals is installed is fixed to the electromagnetic shield member 3 with screws 80. The CRT circuit board 15, which transmits the image signals outputted from the printed circuit board 14 to the CRT 4, is electrically connected to the printed circuit board 14 through braided wires. In FIG. 39, the CRT circuit board 15 is depicted as if it were floating in the air for convenience of explanation; however, actually, a CRT socket, not shown, is attached to the CRT circuit board 15, and during assembling process of the electronic device 150, the CRT socket is inserted into a neck portion of the CRT 4 so that the CRT circuit board 15 is fixed to the CRT 4.
As described above, in the conventional electronic device 150, all the electromagnetic shield members 1, 2, 3, 5, 6 and 10 including the electromagnetic shield members 2 and 10 requiring no mechanical strength have been made of metal plates; therefore, it has the following problems:
First, since screws 80 are used to connect the metal plates to each other, distortion tends to occur at portions in the metal plates fastened by the screws 80, resulting in gaps between the metal plates that are supposed to be connected to each other. In order to prevent these gaps from giving adverse effects on the suppression of electromagnetic interference, it is necessary to narrow the intervals between the portions fastened by the screws 80. Moreover, also in order to implement a desired electrical connection between the metal plates and a sufficient electromagnetic shielding effect, it is necessary to provide a number of fastening portions with screws 80. The resulting problem is that the time required for the screw-fastening job is lengthened, causing high processing costs. Moreover, since a number of screws 80 are required, the material costs also become higher.
Second, in order to reduce the processing costs of the metal plates, it is preferable to carry out the screw fastening process without tapped openings in the metal plate. For this reason, the screw-fastening process is carried out while tapping the metal plate by using the screw 80, with the result that cutting powder from the metal plate tends to fall on the inner circuit of the electronic device 150; consequently, in order to prevent this from causing short-circuiting in the electronic parts, considerable attention or corresponding countermeasures are required.
Third, the electromagnetic shields are generally formed in such a manner as to surround the generation source of electromagnetic waves, and, for example, the electromagnetic shield members 2 and 3 are formed in such a manner as to cooperatively surround the printed circuit board 14 which is a source of electromagnetic waves. For this reason, as illustrated in FIG. 39, the electromagnetic shield member 2 needs to have a complex container shape. In order to provide this shape, complex processes, such as a bending process and a welding process (or a joining process using riveting and caulking jobs), have been required. This also results in high processing costs. In the case of executing the bending process, molds need to be prepared, resulting in a long working time and an increase in the material costs required for the processing. Moreover, prior to transporting the electromagnetic shield member 2 to an assembling site for the electronic device 150, it needs to be subjected to processing such as welding. In comparison with flat metal plates, the electromagnetic shield members 2 after having been processed require higher costs to be transported to the assembling site.
Fourth, in order to improve the electromagnetic shielding effect, generally, it is effective to have a wider area in which the metal plates to be connected are allowed to face each other (or to contact each other) so as to enhance the reflection loss effect of electromagnetic waves; however, this provision needs more materials, resulting in an increase in the material costs.